The objective of this research program is to evaluate the therapeutic value of yeast phenylalanine ammonia-lyase (PAL) and other ammonia-lyases in tumor bearing animals. In a limited series of studies, intraperitoneal administration of PAL gave a significant number of cures of an ascites leukemia maintained in mice, presumably by lowering the levels of phenylalanine in the plasma. Maintenance of mice on a diet low in phenylalanine did not enhance the therapeutic effectiveness of PAL. New methods for the rapid purification of PAL were developed so that large quantities of enzyme may be prepared and evaluated against a wide spectrum of tumors. The long range goal is to develop PAL as a new agent for the treatment of cancer in humans.